When dealing with AC signals, circuit designers usually are concerned with rms voltage values. However, AC measurements typically involve a complex interaction among signal amplitude, harmonic content and phase relationships. With respect to ac motors, fundamental voltage, rather than total rms voltage (fundamental voltage plus harmonics), is the parameter which is adjusted to meet motor nameplate voltage. For example, AC motor torque is a function of fundamental voltage. If the total rms voltage applied to the motor is set so as to be equal to the motor nameplate voltage, the motor will produce a torque less than that which could be safely attained if a fundamental voltage substantially equal to the motor nameplate voltage had been applied to the motor. Similarly, motor speed is a function of the frequency of the fundamental voltage. Thus, by setting the frequency of the fundamental voltage to that indicated on the motor nameplate, the motor speed on the nameplate will be achieved.
Voltmeters for measuring parameters, such as fundamental voltage, are presently available, however, such voltmeters are costly and quite bulky. In addition, the resulting measurements from such voltmeters are quite inaccurate.
Because of the desirability of measuring fundamental voltage and fundamental frequency, and the inherent problems associated with presently available apparatus to measure same, it has become desirable to develop a voltmeter which accurately measures the fundamental voltage of a signal and the frequency of the fundamental voltage.